In a preferred embodiment of the present invention, a properly-formed, thin beam of electro-magnetic radiation, or light filament, is rapidly scanned through space at velocities generally perpendicular and very high relative to the velocities of particles which are transported by fluid movement. The locus of the light filament movement can be described as a volume in space through which the particles pass, and during such passage, the particles generate light scattering signals which are measured by an electro-optical detector. The detection response to the scattering signals is preferably from a limited region, or scattering volume, or sensitive volume. We prefer the designation scattering volume and use it exclusively hereinafter. Whereas, typically, the thickness of the scattering volume is small compared to its transverse dimension, it is large compared to the dimensions of the particles being measured. Illustrative dimensions are transverse 1cm, thickness 0.01cm, and particle size 0.0001cm.
Present day concerns for microcontamination of process fluids, such as gases in semiconductor manufacturing, require the detection of fine particles having diameters in the range of 0.1 to 10 micrometers (0.00001 & 0.001 cm), in very clean gases having perhaps 10-100 such particles per cubic foot (0.0004 to 0.004/cm.sup.3) for illustrative example. To achieve adequate detection sensitivities and counting rates with commonly-available sources of electro-magnetic radiation, such as lasers, it is found that conflicting requirements arise. In very clean environments, the aerosol number density is low. For good statistics one requires large volumetric sampling rates and therefore large scattering volumes. The requirement of large scattering volume leads to larger laser beams and thus to reduced beam intensity and increased minimum particle size detection. It also leads to increased interference from radiation scattered by the far more numerous molecules comprising the fluid by which the aerosols are transported. This also increases minimum particle size detection.
In prior art stationary beam devices, these conflicting requirements of reducing minimum detectable particle size and increasing volumetric sampling rate result in severe problems. As the physical features of semiconductor circuitry necessarily decrease, the particles which will cause a "kill-defect" necessarily decrease both in size and concentration. Hence, prior art instruments do not provide satisfactory information.
The scanning scattering volume of our invention alleviates these problems and, in one preferred embodiment, enables substantially increased volumetric sampling rates while retaining the same minimum detectable size. In another embodiment substantially smaller minimum detectable particle size is achievable while retaining the same sampling rate as prior art stationary beam counters.
In addition, the preferred form of the present invention provides 100% coverage of a conduit or pipe in which the fluid is transported without reducing the diameter of the pipe. Some prior art devices require significant reductions in cross section of the pipe in order to transport the aerosols through a small stationary beam at high speed, leading to size-dependent losses. Other prior art devices do not use flow constrictions but rather electro-optically define a small scattering volume which covers a very small percentage of the cross-sectional area. Accordingly, both such stationary beam device designs necessarily have low true volumetric sampling rate.
Finally, the present invention in its preferred embodiment also provides improvement of the E-0 response function. All electro-optical counters respond imperfectly in the sense that particles of a unique size do not produce precisely unique output responses such as pulses in scattered photon flux, photo current, or voltage. Because the scanning method can produce multiple "hits" upon or "scans" of a particle as it traverses the scattering volume, a better response function can be achieved.
In accordance with a preferred embodiment of the present invention an apparatus is provided for detecting particles in a fluid. The apparatus includes a flow cell and windows are formed in the flow cell for admitting light beam(s) and receiving scattered light signals. A conduit directs a flow of fluid and particles in a path through the flow cell. A source of radiation is provided for producing a radiation beam. A scanning device, preferably a scanning mirror, produces a scanning radiation beam and the scanning beam moves between at least first and second scan positions at a high scan velocity. The scanning beam is directed by an optical system through a window into the chamber and into the path of the fluid and particles. When a particle is scanned over or "hit" by the scanning beam, a scattered light signal is produced. This light is collected by an optical system and is caused to fall on a photosensitive detector which produces a detection signal corresponding to the scattered light. In the preferred embodiment, the detection signal is analyzed by a computer to determine the concentration of particles passing through the scattering volume and to further determine the size and velocity of each particle that is counted.